Allow switching between specialisations.

[daschw]

Prefer switching to specialisation provided as --specialisation argument over current specialisation.

---

Currently, assuming a config like in the README

{config, pkgs, ...}: {
  specialisation."foo".configuration = {
    environment.etc."specialisation".text = "foo";
    # ..rest of config
  };

  specialisation."bar".configuration = {
    environment.etc."specialisation".text = "bar";
    # ..rest of config
  };
}

we have this - in my optinion - unexpected behaviour:

nh os switch -s foo # switch to foo
nh os switch -s bar # switch back to no-specialisation

---

After these changes

nh os switch -s bar

and

nh home switch -s bar

switch to the bar specialisation regardless of the current specialisation.

[daschw]

I guess this will be fixed by #281 anyways